1. Field of the Invention
The present invention relates to a field effect transistor (FET)-type biosensor including a source electrode, a gate electrode, and a drain electrode, and more particularly, to an FET type biosensor in which the surface of a gate is modified to increase sensitivity.
2. Description of the Related Art
As human DNA sequences become known through the completion of the genome project, research into the functions of genes and proteins encoded from genes are being more actively conducted. In response to this active research, the need for development of biosensors that can easily detect biomaterials has increased.
Biosensors that can detect biomaterials using an electric signal are disclosed in U.S. Pat. Nos. 4,238,757; 4,777,019; 5,431,883; 5,827,482; and the like. In particular, U.S. Pat. No. 4,238,757 discloses a field effect transistor (FET) including a source and a drain which is designed to have an antigen reacting to a specific antibody. In this case, the change in the concentration of the antigen in a solution with respect to a drain current was observed over time using the FET.
An FET disclosed in U.S. Pat. No. 4,777,019 includes a gate formed on the doped source and drain regions, and a complementary nucleotide with a nucleotide to be measured is affixed to the gate.
An FET disclosed in U.S. Pat. No. 5,431,883 includes a phthalocayanin thin layer connecting a gate to a drain. Phthalocyanin is an organic insulating material that can be transformed into a conductive material by reacting a chemical sample.
U.S. Pat. No. 5,827,482 discloses a biosensor which includes two FETs which includes molecular receptors bound to gates. The two FETs are arranged in a row to increase sensitivity to different bindings.
FIG. 1 is a schematic diagram of a conventional FET. Referring to FIG. 1, a target oligonucleotide is affixed to the surface of a gate of the FET. FIG. 2 illustrates an increase in the charge accumulation due to binding between an oligonucleotide and a desired complememtary DNA wherein the oligonucleotide is affixed to the surface of the gate of the FET shown in FIG. 1. FIGS. 3A and 3B illustrate the effects of variations in debye length in ImmounoFET. As the size of a molecule affixed to the surface of the gate increases, it is difficult to detect a signal within debye length. Referring to FIG. 3B, when the ionic strength is high, debye length (dAb) is small, and thereby, a signal resulting from a reaction between Ag and Ab is difficult to detect, but when the ionic strength is low, debye Length (dAb) is large, and thus, the signal resulting from the reaction between Ag and Ab can be detected.
Conventional techniques are based on the structure in which an oligonucleotide is affixed to the surface of the FET gate, and such a structure is often used in conventional microarrays. Although the structure does not cause any problems in the microarray, it does in an FET sensor. That is, in the FET sensor, a depletion region is generated from hybridization occurring in the surface of the gate and only charges within the debye length can derive the formation of the depletion region. In order to obtain an effective depletion region, DNA can be arranged parallel to the surface of the gate so that the contact area between DNA and the gate is increased. The parallel structure provides better results when DNA is arranged perpendicular to the surface of the gate. However, surface fixation methods often used in microarrays are used in conventional FET sensors, and thus, an effective depletion region is difficult to obtain. In addition, the fixation of the target oligonucleotide to the surface of the gate results in many disadvantages. For example, when a probe is fixed on the surface of the gate, a long time is required to fix the probe and to perform hybridization in a solution. Typically, DNA hybridization occurs in a solution, but in FET sensors a buffer solution with low ionic strength is used to hybridize DNA to maximize the debye length. The buffer solution with low ionic strength may be preferred for sensors, but results in decreased DNA hybridization efficiency. That is, the hybridization occurring on the surface of the gate is decreased and low signals are generated.
When attempting to overcome these problems in conventional techniques, the inventors of the present invention have confirmed that when the surface of a gate of an FET sensor is modified, a DNA backbone is adsorbed at the surface of the gate, and signals can be effectively detected within the debye length.